The hydraulic cylinder is an actuator that converts hydraulic energy into mechanical energy. It produces linear motion and imparts a force that depends on the pressure of the oil and on the area of the piston. It has many applications in oil hydraulics systems, and is employed for example in earth moving machines, cranes, presses, industrial machinery etc.
The device is composed of a cylindrical housing (also called bore or barrel), a rod with a piston, closed by a cap on both ends. With the term “tubes for hydraulic cylinders” we mean the tubes for the production of the external cylindrical housing, which is common to all types of hydraulic cylinders, see e.g. FIG. 1.
Technical requirements of this product can be reassumed in the following way.                To ensure proper transmission of force and to avoid losses of the hydraulic medium, the barrel must have good toughness and narrow geometric tolerances in the inner diameter. If these high precision characteristics cannot be directly or almost obtained through the metallurgic production process of the seamless pipe employed for the barrel, downstream machining operations comprising, in this case, highly ablative surface treatments (e.g. skiving plus roller burnishing or honing or boring plus honing) are necessary. Importantly, the former machining step increases the production costs sensibly, since the highly ablative treatments must be followed in their turn by a (stepwise) surface refining, to equalize the newly created surface. In general, the most economic solution is the process of skiving and burnishing, that requires precise and repeatable dimensional tolerances. If these conditions are not met, more expensive solutions must be adopted, for example boring plus honing or boring plus skiving and burnishing.        
It follows thus that the final machining costs increase in an over proportional manner with growing geometric tolerances.                The barrel undergoes fatigue cycles during its life and on top of that, in many applications such as its employment in earth moving machines, cranes and others, it must be able to operate in external conditions of low temperature. Toughness (at least down to −20° C. and preferably down to −40° C.) is therefore an essential requirement to have “leak before break” behaviour, avoiding in this way brittle fracture, which typically involves a dangerous condition. Indeed, for a number of applications such as pressure equipment, the Laws already demand ductile behaviour in burst tests, or longitudinal and transversal toughness of 27 J at the minimum of the operating temperature [1,2,3].        
The manufacturing process of the cylinder barrel is economically more advantageous using a cold finished tube instead of a hot rolled tube, due to the possibility to get:                Dimensions closer to the final size, with narrower tolerances, thus making the downstream machining process, if any, comparably cheap, due to the only very limited amount of dimensional correction required.        Higher tensile properties.        Better surface quality.        
The standard cycle is, therefore:                Hot rolling-pickling-cold drawing-stress relieving-straightening-surface machining-cut-assemblage of the parts.        
In the standard cycle, cold drawing and stress relieving are necessary to increase the yield strength to the levels commonly required (at least 520 MPa, preferably 620 MPa), but they reduce material toughness and more importantly they cause a high anisotropy between longitudinal and transversal direction of the tube, in particular to the detriment of transversal toughness. Therefore, with the standard cycle, it is not possible to ensure the low temperature characteristics required e.g. by applications in specific climatic conditions as they may be encountered e.g. in northern Europe. Indeed, in such cases even at room temperatures the transversal toughness is not enough in order to avoid brittle fracture.
The alternative cycles today available to improve the toughness at low temperature are:
(1) Hot rolling-cold drawing-normalisation-straightening-surface machining-cut-assemblage of the parts.
This solution lowers, however, the tensile properties (yield strength), so a higher wall thickness is necessary to operate at the same pressure, increasing weight and thus energy consumption related to the operation of the respective equipment.
(2) Hot rolling-quench and temper-straightening-surface machining-cut-assemblage of the parts.
(3) Hot rolling-pickling-cold drawing-quench and temper-straightening-surface machining-cut-assemblage of the parts.
In both of these cases (2), (3), surface quality and tolerances don't reach the standard required by the market for seamless precision tubes and thus require particularly expensive highly ablative downstream machining operations. Case (2) requires a preventive and consistent material removal through a boring operation, followed by skiving and burnishing or honing. In case (3) geometrical variations and distortions induced by martensitic transformation increase ovality and variability of the diameters, affecting the repeatability and the advantage of producing a precision steel tube. The treatment of Q&T also increases the production cost.
This means that, so far, either (i) the use of high wall thickness or (ii) the expense of high production costs is necessary to improve the low temperature performance of hydraulic cylinders.
In an effort to arrive at a production process not displaying the drawbacks of the cycles (1)-(3), an alternative cycle has been adopted in the past.
(4) Hot rolling-normalization (or on-line normalising)-cold drawing-stress relieving-straightening-surface machining-cut-assemblage of the parts.
While cycle (4) is advantageous from the point of view of the production costs, it guarantees nevertheless good longitudinal toughness only at room temperature and a sufficient one at 0° C. At temperatures below zero degrees, the variability of the process becomes too high and it's difficult to obtain consistent values. The transverse toughness is, on top of that, often unsatisfactory.
This means that cycle (4) does not improve the safety of the hydraulic cylinder, except in warm climatic conditions.
Hence, there remains an urgent need in the art for the provision of new seamless precision steel tubes with improved isotropic toughness at low temperature for hydraulic cylinders. Desirably, at a working temperature of −40° C.—reflecting usual conditions in specific areas of the planet—the minimum isotropic (i.e. longitudinal and transversal) toughness should be higher than the prescribed threshold limit of 27 J. On top of that, there remains an urgent need in the art for the provision of a new process for obtaining the aforementioned new tubes, the said new process being less expensive than the known cycles (1)-(4) as above.
The new process should be able to employ common low carbon steels, with a minimum content of Mn and Si, and possibly, but not necessarily micro-alloyed with one or more of the further elements, such as Cr, Ni, Mo, V, Nb, N. Al, Ca.